Resin impregnated fibrous sheet dried to the B-stage (prepreg), especially epoxy resin impregnated glass cloth, has been used in manufacturing high-pressure laminates for the printed circuit board industry. Usually, an epoxy resin-curing agent system, dissolved in an organic solvent to provide a resinous solution, is impregnated into a passing fibrous sheet in a closed-bottom resin bath. The impregnated sheet is then generally passed through a drying oven to B-stage the resin, i.e., remove sufficient solvent so that the resin is dry to the touch, capable of being wound on a reel or cut into sheets and stacked for storage without blocking, and also capable of further cure to the completely cured C-stage. Such processes are taught by Alvino et al., in U.S. Pat. No. 4,327,143, and Thomas et al., in U.S. Pat. No. 4,477,512.
Recently, stringent restrictions on volatile organic compound (VOC) emissions have curtailed the use of some resinous impregnating solutions. The prepreg prepared by a conventional web-coating process also has problems with air being trapped in the prepreg. A void-free prepreg is necessary for producing a void-free laminate with optimum electrical, and physical properties. A variety of vacuum-resin impregnation processes, such as those taught by Russel, in U.S. Pat. Nos. 3,616,194 and 3,826,703; Fox et al., in U.S. Pat. No. 3,644,137; and, Nothiger, in U.S. Pat. No. 3,881,445, have attempted to eliminate air trapped in a fibrous sheet prior to sheet impregnation. Most of these processes, however, are complicated and expensive, and many require vacuum-tight seals. Other means of impregnating woven and non-woven flexible materials with high solids, solventless resin systems are needed.